Temporal effect of straw addition on amino acid utilization by soil microorganisms

Microorganisms may take up simple nitrogen (N) containing organic molecules directly or they may deaminate them first and take up the ammonium released. The present study was carried out to investigate how the incorporation of straw into soil affects the N uptake pathway of soil microorganisms. Soil samples were incubated for 84 days at 20 °C with and without straw addition. The mineralization rate of amino acids was determined using 15N-labeled amino acids. The addition of straw resulted in net N immobilization and the depletion of the mineral N pool during the first week. Added amino acids were either mineralized or taken up by soil microorganisms within 24 h of their addition. The proportion of added amino acids mineralized during the first week was low, suggesting that 70–95% of the added amino acids were taken up directly. The need for amino acids for the synthesis of proteins for the growing microbial community likely favored their direct uptake. After the first week however, the mineralization rate increased and the direct uptake accounted for only 60–70%. Our results suggest that the direct uptake of organic N was generally the preferred pathway, but that the extracellular ammonification may be an important alternative when N is severely limiting.

► Direct uptake was the preferred pathway amino acid N utilization. ► Extracellular deamination became more important when N was severely limited. ► The N utilization pathway was determined by N and C availability. ► Both pathways were active concurrently.